The goal is to partially restore the Sacramento-San Joaquin Delta environment and stop water exports from shrinking. But uncertainty over how much water the delta can spare is likely to last years.

One thing stood out in the pile of documents released Thursday detailing state plans to replumb California's water hub: Construction could start on the massive project before water managers know whether it will work as intended.

The still-evolving proposal, backed by Gov. Jerry Brown's administration and the federal government, is designed to partially restore the Sacramento-San Joaquin Delta environment and halt reductions in delta water exports.

But uncertainty over the volume of future water deliveries is likely to linger for years as government scientists try to nail down how much water imperiled salmon and smelt need in the delta.

"This plan does not include any guarantees for water supply deliveries," said Mark Cowin, director of the state Department of Water Resources.

Proponents also don't know whether restoring about 100,000 acres of habitat in the much-altered delta will produce the desired effect of bolstering fish and wildlife populations.

But state officials argue that doing nothing will guarantee the continued deterioration of the delta ecosystem, and with it, additional cuts to southbound water deliveries.

The more than 1,000 pages released Thursday by the California Natural Resources Agency covered only part of a plan that has been under discussion for years.

It would change the way supplies are diverted from the delta by constructing three large intakes on the Sacramento River that would feed into two 35-mile tunnels, each about four stories tall. The tunnels, burrowed more than 150 feet beneath the delta, would carry water by gravity to existing export pumps in the south delta.

The new facilities would cost $14 billion, which would be paid by water users, including Southern California agencies and San Joaquin Valley irrigation districts that depend on the delta for part of their supplies.

Restoration of delta wetlands, flood plains and wildlife habitat would cost an additional $4 billion, most of which is expected to come from federal and state funds.

A final decision on the project is more than a year away. The resources agency will roll out the rest of the draft plan over the next two months. The proposal still has to obtain environmental approvals, and federal and state fish and wildlife agencies have to determine the conditions under which the system would operate.

Those conditions, aimed at protecting endangered species, would determine the volume of delta exports. The resources agency said deliveries could be 10% less than the average of the last two decades — or 5% more.

Reactions to the plan echoed previously voiced support and criticism.

"There's a rush to build it first and then test it," said Zeke Grader, vice chairman of the Golden Gate Salmon Assn., a consortium of commercial, recreational and tribal fishermen that says the new system would harm migrating salmon.

Water and irrigation districts that have suffered cutbacks in delta supplies called the plan a landmark.

"California's water delivery system is broken and the [plan] is the best option our state has in securing a reliable water future," said Mike Wade, executive director of the California Farm Water Coalition.